Power supplies for electronic equipment can fail. A common approach to protecting against such failures is to provide a battery back-up. One type of battery back-up is shown in FIG. 1.
In this back-up system, a diode is connected from a back-up battery to the equipment. The diode polarity is such that, when the power supply voltage, V.sub.POWER, is normal, the diode is reverse-biased (because the battery voltage, V.sub.BATTERY, is below V.sub.POWER), and thus the diode acts as an open circuit. However, when V.sub.POWER drops below V.sub.BATTERY by a sufficient amount, such as 0.7 volts, the diode becomes conducting, and the battery now supplies power to the equipment.
While this approach works well in many situations, it does have disadvantages. For example, a voltage drop exists across the diode, such as the 0.7 volts (for a silicon device) discussed above. Consequently, if it is desired that the back-up voltage which is actually delivered to the equipment remain equal to V.sub.POWER, the battery must deliver a voltage which is high enough to compensate for the voltage drop across the diode. In this example, V.sub.BATTERY must exceed V.sub.POWER by 0.7 volts. Batteries having such fractional ratings can be expensive and hard-to-find.
Further, it is not even clear that such an arrangement is workable. That is, if V.sub.BATTERY exceeds V.sub.POWER by 0.7 volts, the diode will be conducting even when the power supply is functional. To inhibit the conduction of the diode, the differential between V.sub.BATTERY and V.sub.POWER must be reduced or eliminated. In practice, many designers reduce the differential by setting V.sub.BATTERY equal to V.sub.POWER. However, the reduction also reduces the back-up voltage received by the equipment: when the power supply fails, the voltage delivered to the equipment will equal V.sub.BATTERY minus the diode drop. If V.sub.BATTERY equals 9 volts, then the voltage delivered to the equipment upon power supply failure will be about 8.3 volts. This lowered voltage can be undesirable in some cases.